FIG. 1 of the accompanying drawings illustrates diagrammatically a known type of receiver arrangement for receiving digital terrestrial broadcast television signals and signals transmitted via digital cable systems. The receiver comprises a radio frequency (RF) input 1 connected to a single conversion tuner 2 for selecting a desired channel for reception and converting it to a non-zero intermediate frequency (IF). The output of the tuner 2 is connected to an IF stage 3, which provides variable gain in accordance with an automatic gain control (AGC) arrangement and channel filtering in the form of a surface acoustic wave filter (SAWF) for passing the selected channel at intermediate frequency and for attenuating other channels which may be present in the output of the tuner 2.
The output of the IF stage 3 is connected to a demodulator 4. The demodulator comprises an analog/digital converter (ADC) 5 which converts the selected channel at intermediate frequency to the digital domain. The output of the converter 5 is supplied to a demodulator block 6 which, in the case of coded orthogonal frequency division multiplex (COFDM) signals, principally comprises a demodulator and a fast Fourier transform (FFT) stage. The output of the demodulator 6 is supplied to a forward error correction (FEC) block 7 which performs the appropriate error correction, such as Reed Solomon or Viterbi correction. The demodulated error-corrected data are supplied, for example as an MPEG transport stream, to the output of the demodulator 4 for further processing by a baseband section (not shown) of the receiver.
The single frequency changer in the single conversion tuner 2 converts the selected channel to a standard non-zero intermediate frequency. In the case of digital terrestrial television receivers and digital cable receivers, three intermediate frequencies are in common use nowadays. 36 MHz is used, for example, for COFDM modulation in Europe. 44 MHz is used, for example, for VSB (vestigial sideband) modulation in USA. 57 MHz is used, for example, in Japan. In such receivers, it is necessary for adequate performance to be achieved in the presence of the image channel. It is usual for “high side mixing” to be employed, where the local oscillator frequency is above the desired channel. In this case, the image channel in the radio frequency domain will be at twice the intermediate frequency above the selected channel. For example, in the case of receivers with an intermediate frequency of 36 MHz, the image channel is 72 MHz above the selected channel. When translated to radio frequencies in the terrestrial television UHF band from 470 to 860 MHz, the frequencies affected by the image frequency are from 470 MHz to about 788 MHz. It is also possible in terrestrial transmission broadcasting for the image channel to have a substantially greater amplitude than the selected channel. The image channel may be as much as 46 dB or more above the selected channel. The image channel must therefore be suppressed to a level where it does not cause significant interference with the desired or selected channel and this is generally achieved by applying image filtering in the tuner 2.
In the case of known single conversion tuners such as 2, image suppression is achieved by the use of tracking RF filters between the input 1 and the frequency changer of the tuner 2. Such filters are typically of double-tuned type having a passband which is tuneable so as to be centred on a selected channel. Thus, the passband of the centre frequency is required to follow or “track” the frequency of a local oscillator of the frequency changer with an off-set such that the local oscillator frequency is greater than the centre frequency of the filter passband by the intermediate frequency in high-side mixing.
Tracking between the local oscillator frequency and the radio frequency filter passband centre frequency is achieved using suitably matched components such as varactor diodes within the local oscillator resonant network and the tracking filter. However, such components are subject to normal manufacturing tolerance spreads so that the filter and the local oscillator do not generally track acceptably across the transmitted frequency spectrum without an alignment step during manufacture. Accordingly, the resonant network and/or the filter includes at least one adjustable component which is aligned during manufacture so that the oscillator and filter frequencies vary as required to an acceptable tolerance when tuning across the transmitted spectrum. Typically, free standing wire wound or metal plate inductor elements (“air-coils”) are used and alignment during manufacture involves mechanically adjusting such air-coils, for example changing the spacing between coils or plates or changing the mechanical dimensions of plates or coils from their original PCB (printed circuit board) assembled dimensions, so as to achieve acceptably accurate tracking of the local oscillator frequency by the image suppression filtering.
Such tuners are not attractive for implementation directly on to the main PCT (mother board) of a digital receiver because such mother boards are expensive and any mechanically adjustable component adds significant cost to the assembly and testing of the mother board. The production yield is reduced because of increased production test failure due to mechanical damage to the air coils by alignment operators or robots. It is also more difficult and expensive to service products with such mechanically adjustable air coils as this would require not only coil or plate replacement but also full mechanical re-alignment of the air coils using the entire mother board.
U.S. Pat. No. 5,949,832 discloses an arrangement for use in a digital data receiver for controlling the bandwidth of I and Q channel filters following frequency conversion of an input signal in a quadrature down-converter. The filters are referred to as Nyquist filters, which implies that the down-converter is of the zero intermediate frequency type and that the filters are low pass filters of controllable turnover frequency. The filter bandwidths are controlled so as to adapt to different data rates.
GB2364455 discloses the use of an active control loop for controlling the bias current of an input radio frequency amplifier to achieve an acceptable bit error rate.
GB2367700 discloses the concept of automatic alignment of tracking radio frequency filters without disclosing any technique for achieving this.